SOAP is an acronym for Simple Objects Access Protocol. Response uses SOAP over HTTP and HTTPS to transport secure messages. You must have a license to use SOAP and Response Secure Messaging. These products will be listed under "Licensed Products"` if you have the corresponding license.
SMTP is an acronym for Simple Mail Transport Protocol, which has been the main Internet protocol for sending e-mail.
You can select either an SMTP or SOAP server. Each can be used as a trusted channel to carry secure messages, but typically the SOAP server is used for Secure Messaging. Often SMTP servers are untrusted because the message often passes through untrusted public networks without guaranteed confidentiality.
Response now supports secure SMTP servers (SSL). Review the About PCI Compliance topic where you will also find links to related topics.
There are two channels associated with SOAP:
The Response mail receiver supports an inbound SOAP channel.
The Response mail sender supports an outbound SOAP channel.
If the Response outbound server is not specified, Response assumes that it is the same as the inbound server.